A body sensor network (BSN) is a network of devices that communicate wirelessly with one another and are located in the body or on the body of a person like a patient, or in the immediate vicinity of the body. Due to the convenience enabled by the absence of cables, BSNs are increasingly being used for patient monitoring as well as for many other applications. A typical BSN, as used for patient monitoring, consists of a set of on-body sensors and one off-body monitoring device that receives the vital sign data measured and transmitted by the sensors. The monitoring device is usually located within a distance of less than five meters from the patient's body.
It is preferred that patient monitoring BSNs communicate at frequencies higher than 2 GHz because those frequency bands are especially suitable in terms of licensing costs, data transmission rate and antenna size. Nevertheless, the high attenuation introduced by the human body at frequencies above 1 GHz is the main challenge for reliable BSNs. Since high RF frequencies barely propagate through the body, parts of the person's body often block out the direct RF propagation path—i.e. the visibility or line-of-sight (LOS)—between BSN devices. Although two devices without LOS can often communicate, the non-LOS link between them is much less reliable than a LOS link.
Wireless communication via non-LOS is still possible because of two effects: multipath propagation and creeping waves. Thanks to multipath propagation a device without LOS receives multiple reflections of the signal transmitted by another device. Such reflections originate when the transmitted RF signal bounces off the user's environment, e.g. floor, walls, furniture, etc., and hence heavily depend on it. The creeping waves effect can be understood as a diffraction or waveguide effect that causes the RF signal of a device to propagate following the contour of the body. Both multipath propagation and the creeping waves help on-body BSN devices to communicate. Nonetheless off-body devices such as a patient monitor cannot benefit from the creeping waves and depend solely on the reflections from the environment.
As set out above, conventional body sensor networks, also known as body area networks (BANs), are not reliable enough for health monitoring, mainly due to the poor RF propagation conditions introduced by the proximity of the human body. Although the attenuation problem is especially detrimental for medical BSNs it has also been identified in other kinds of networks and for other application domains. Most conventional approaches to address this problem are reactive approaches based on first monitoring the conditions on the wireless link using packet or bit error rates (PER and BER), received signal strength, or other signal quality metrics, and, then, performing countermeasures whenever the link conditions become adverse. Such countermeasures include the following:
A first conventional approach is to use an alternative link that was assessed to have better conditions to deliver data. Thus a device sends its data to an intermediary device instead of to the destination device. Most packet routing protocols are based on this. Further, another conventional approach is increasing the transmit power to ensure a better signal at the receiving device. This is known as dynamic link adaptation, dynamic power management, or dynamic link power control. A further conventional possibility is decreasing the data rate with which information is physically transmitted in order to decrease the transmission error probability. For example, this mechanism is used in wireless technologies, and for example referred to as dynamic rate scaling.
Other approaches apply precautionary measures independently of the actual conditions of the wireless link. Some of the most relevant are the following:
Antennas optimized for on-body operation can be used in order to reduce the magnitude of the attenuation. Further, the transmit power can be increased permanently in order to ensure a better signal at the receiving device. Furthermore, using flooding-based packet routing so that every device forwards the packets received from all its neighboring devices is another conventional possibility. Since the same information is sent via multiple routes in parallel, it is more probable that it arrives at its destination.
However, the conventional state-of-the-art approaches to tackle the RF attenuation issues are only partially successful and exhibit important disadvantages. For instance, the first three approaches mentioned before cannot completely prevent the loss of information because they are based on reactive measures and cannot anticipate a degradation of the link conditions. In other words: They typically react to a problem when it has already occurred. Precautionary approaches are not much more successful: While the fourth approach mentioned above enables only a minimal improvement of the wireless link conditions, the fifth approach dramatically reduces the operating time of the BSN and the sixth approach quickly overloads the wireless channel.